In an attempt to find a quantitative, direct technique for measuring myocardial ischemic injury, we evaluated the relationship between the pH of ischemic myocardium and the severity of myocardial ischemic injury. We measured intramyocardial pH (pHm) for 15 min. using specially designed fiber-optic pH probes recently developed at the NIH, and myocardial levels of ATP and lactate 20 min. after coronary occlusion in 20 open-chest dogs. We found good correlation between 15 min. pHm and 20 min. ATP (r = 0.80), and 20 min. lactate (r = -0.88). Although a close correlation between pHm and either of these myocardial metabolites would not always be expected, a good correlation confirms that pHm is a useful indicator of myocardial ischemic injury in the early phase of ischemia following coronary occlusion, as assessed by ATP and lactate levels in the myocardium. Thus, with measuring pHm, the degree of myocardial ischemic injury can be evaluated readily in the intact, beating heart.